Biomedical implantable devices have been developed using functional electrical stimulation (FES) techniques for restoring lost or diminished neurological functions or muscular disabilities. FES characteristically utilizes current pulses applied to target nerves or reflex centers in programmable patterns and sequences by means of electrical stimulators. To compensate for nerve and tissue impedances, a high output voltage is typically required for the electrical stimulators. Since many implantable devices are powered by means of inductive coupling between an internal coil and externally applied magnetic fields, techniques have been developed to generate a sufficiently high output voltage in order to accomplish successful FES. One common technique is to generate a high AC induced voltage on a coil such that a high output voltage can be obtained by the use of linear regulators configured to regulate the AC induced voltage (see for example, K. Chen et al., “An integrated 256 Channel Epiretinal Prosthesis”, IEEE Journal of Solid State Cir., vol. 45, pp 1946-1956, September 2010). Other circuits in the implant may also require reliable regulated voltages, typically low voltages, which would require additional regulators coupled to the high voltage output. The net result of such approach is a system that is usually not power efficient due to losses on the regulators and rectifiers especially when the low voltage circuits are consuming large amounts of power.
In another approach to generating a high output voltage, an induced AC voltage with an amplitude sufficiently high to supply the requirements of low voltage circuits is used to generate a low regulated voltage which is then up converted using a switched capacitor DC-DC converter or a boost converter (see for example X. Zhang and H. Lee, “An Efficiency-Enhanced Auto-Reconfigurable 2x/3x SC Charge Pump for Transcutaneous Power Transmission”, Proc. of IEEE, CICC, pp 311-314, September 2009). In addition to the power loss in the rectifier using this approach, the switched capacitor DC-DC converter may also have high power losses during voltage regulation when generating different output voltages. Additionally, when a boost converter is used, a bulky inductor necessary for converter operation is required to fit inside the implant which typically has severe size constraints and therefore limits the applications for the use of such converters. Accordingly, in overcoming the deficiencies of the currently available circuits and approaches, a capacitor based AC-DC step up converter is disclosed for the generation of output voltages. Uniquely, multiple output voltages are generated to satisfy simultaneous requirements for high voltage stimulator applications along with requirements for low voltage stimulator applications. Therefore, the overall power dissipation, when considering all of the stimulators, can be minimized when connecting the stimulators to the appropriate and different output voltages.